In circuit technology, and especially in integrated circuits, it is desirable to convert an input voltage to an output current without the introduction of errors in the conversion. One such arrangement for accomplishing this result is to use an operational amplifier, a transistor and a resistor. However, this arrangement has disadvantages since the op amp must have a negative voltage source and since it requires a relatively large semiconductor area in integrated circuit applications.
Circuits not requiring an operational amplifier have been devised, for example, the circuit of VanZenter and Huijging shown in the IEEE Journal of Solid State Circuits, Vol. SC-10, No. 6, December 1975, page 432, for converting differential voltages to a current. Such circuits have a non-linearity problem since the current through the resistor causes the input transistor base-emitter voltage to rise while the output transistor base-emitter voltage falls unequally. In essence then, the prior art circuits suffer from the problem that there is a voltage drop across the various transistor junctions, which voltage drop is logarithmically related to the current. In such situations, the temperature also plays a large role in making the circuit inaccurate.